Severe sepsis leads to systemic inflammation, wherein coagulation and complement activation play critical roles. Evidence from patients and animal models suggest that sepsis is a multi-stage, multi-factorial disease in which the early fulminate inflammatory response to the invading bacteria leads to hypo-perfusion and ischemia-reperfusion (IR) injury that evolves to multiple organ failure (MOF) and ultimately to death. Work from our lab has demonstrated that severe sepsis induced in animals by using sublethal doses of E. coli develops as two-stage series of events, each stage being driven by different pathophysiologies. First stage events are caused by the direct effects of the pathogen, whereas the second stage occurs as result of an aberrant host recovery after IR. The objectives of this proposal are to investigate whether inhibition of complement activation alone or as combination therapies could prevent MOF and improve the outcome of sepsis. Aim 1 will use a C3 convertase inhibitor to determine the role of complement during each of the two stages of sepsis, assess complement activation as a potential cause of thrombocytopenia in sepsis and identify organ- specific protective effects of complement inhibition during sepsis progression. This research will employ cDNA microarray, immunocytochemistry, electron microscopy and biochemical analysis of vital organs to identify the effect of complement activation products on the dominant pathophysiologic processes controlling the progression of sepsis. Aim 2 will determine if, and through what mechanisms, complement inhibition by the recombinant lectin-like domain of thrombomodulin (TM-LLD) could prevent organ failure and improve outcome in our sepsis model. Aim 3 will determine whether combining complement blockade with immunological inhibition of CD14 or extracellular histones could provide superior/additional effects therapeutic effects on E. coli sepsis as compared to complement inhibition alone. This project has potential to advance our understanding of the role of complement activation in sepsis progression and to test whether complement inhibition could be used as effective therapy for sepsis-induced organ failure. )